University of Vermont
Department of Plant and Soil Science

Spring
News Article

PRUNING EVERGREENS IN THE LANDSCAPE

Dr. Leonard Perry, Horticulture Professor
University of Vermont

Proper pruning is important for good plant health. Pruning at the
wrong time, or the wrong way, can result in leaf and plant injury. A
University of New Hampshire publication (extension.unh.edu/resources) on
this same topic gives some excellent tips.

Thoughts of pruning really should begin when buying and placing
plants. If you want a plant with a certain shape, try to find a
selection that will grow into that shape. This will save you having to
prune it at all, or mean pruning it minimally. Place plants with
enough room to grow as they mature. Often we don't, and so pruning is
needed to keep crowded plants within bounds. This is most common with
foundation plantings, planted too close to buildings for future growth, or
that grow too tall and block windows.

Not all evergreen landscape plants are pruned the same way. Evergreens
are those that keep their foliage year round. Narrowleaf species are
just that--they have narrow needle-like leaves. Ones such as pine,
spruce, and fir only grow in the spring and early summer, so don't need
yearly pruning. Others such as arborvitae, juniper, yew,
and hemlock grow through the season, so may need yearly pruning.

Broadleaf species have, as the name indicates, broad leaves. They are
more subject to winter injury than the narrowleaf species, so are less
common in the far north. The most common is the rhododendron, while
farther south one sees holly, mountain laurel, and boxwood.
Broadleaf species may need little pruning if sited properly, or every three
years at most.

Early spring is the best time to prune the species with needle leaves, after
the ground has thawed, but before roughly the middle of June. Pines on
the other hand ooze sap, or "bleed", when cut in early spring while the sap
is flowing more. Prune pines in late spring to minimize such
bleeding. Prune broadleaf evergreens in spring after bloom, but if
blooms are not an issue then you can prune in early spring. Avoid
pruning any evergreens, except to remove injured branches, in late summer or
fall. The wounds won't heal as readily, and new growth may be
stimulated. Such tender growth may not harden properly before winter and be
injured.

If you have overgrown established plants, do not try to correct years of
neglect in one pruning or season. The plants will be unsightly, and
may take years to recover if they do at all. Evergreens also should be
pruned less than deciduous trees and shrubs, as they grow slower.
Yews, hemlock, and arborvitae can be pruned harder (more growth taken off)
than juniper, spruce, pine, and fir. This is because they can grow
from dormant buds on older wood.

Especially with the narrowleaf evergreens, many gardeners prune them yearly
by trimming the outer edges, removing a uniform amount from all
branches. This actually is not pruning but is termed "shearing", just
as one would shear a sheep. It has several disadvantages including
loss of the natural shape, reducing the total leaf area, preventing sunlight
from reaching the inside and so creating a "dead" zone, creating a less
structurally strong plant, removing new growth while keeping the older and
less vigorous branches, and creating stale air inside the shrub which is
conducive for insects.

Instead of shearing, thinning and renewal techniques should be used.
These promote more internal growth, reduce winter injury, and promote the
natural form of plants. Selectively removing certain branches will
result in air and sunlight being able to reach the center of the
plant.

To shape a shrub, first envision, or mark with string, a perimeter
line. This is the line branches shouldn't cross. The line should
create a shape wider at the bottom than the top for the most light to all
branches. Then reach into the shrub and remove at a node where
branches join:
*any branches than extend beyond this perimeter line
* no more than one of every four shoots on average
*competing upright vertical shoots, or leaders (upright branches competing
to be the main trunk), and crossing or rubbing branches.
*a shoot that has another nearby that can grow to fill the space
*branches that are growing sideways or towards the shrub center, and
*club-like growth that has resulted from years of shearing.

If you have an evergreen tree, such as a large pine or spruce, that has
gotten too tall or large for its space, you might consider taking it down
and replacing with a small tree or planting. If a small tree, you
might be able to cut it down yourself, otherwise seriously consider hiring
an arborist. This is especially true if the tree is near power
lines.

If cutting down a tree, make sure it will fall the direction you want, and
won’t harm nearby buildings. If you’re merely removing some branches
that have died, are crossing and rubbing, are in the way, or just to let
more light in, do so safely. Use a pole saw for high branches, use
proper protection such as for head and eyes, and use particular caution if
working from a ladder.

Finally when pruning, make sure to use sharp tools to avoid tearing the
bark. Sharp cuts will heal more quickly. Invest in a good file
(ones with diamond bits are excellent), available online or at complete
garden stores, for quickly and regularly sharpening the blades of pruning
tools.